Synchronous optical networks (SONETs), which provide very high data rate fiber optic links for communications, require low phase noise local oscillators for clock recovery. Phase noise, and the resulting effect of signal edge jitter in the local oscillator output, limits the clock speed or pulse rate for clock recovery by contributing to the required pulse width or duration for accurate operation. Additionally, the local oscillator employed in such applications should be frequency-tunable, allowing the local oscillator to be set or adjusted to a specific frequency to, for example, track frequency variations in the received clock signal. However, maintaining low phase noise and providing significant tune range for a local oscillator have proven to be conflicting objectives.
The related application identified above discloses a two port surface acoustic wave (SAW) resonator for local oscillators which provides both low phase noise and wide tune range (as compared to prior art oscillators employing SAW resonators). However, a local oscillator of the type disclosed—when employed, for example, for clock recovery in SONET applications—is typically mounted on a printed circuit board in close proximity with a number of digital devices operating at clock speeds equal to or greater than 1 gigahertz (GHz). Noise emanating from such digital devices can interfere with operation of the local oscillator regardless of how low the internal phase noise is within the local oscillator.
There is therefore a need in the art for low phase noise local oscillators tolerant of hostile environments.